Process for burning lumps of material with lean gas

ABSTRACT

A method for burning lumps of material, in particular limestone, is carried out in a regeneration shaft kiln ( 3 ) with at least two kiln shafts ( 1, 2 ) interconnected by an overflow line ( 19 ) and in which the material to be burned passes through an upper pre-heating zone ( 8 ), a central combustion zone ( 18 ) and a lower cooling zone ( 14 ). Combustion gas with a low calorific value of less than 7.5 megajoules (MJ) per m 3  is periodically and alternately supplied to each of the shafts ( 1, 2 ) through burner lances ( 9, 25 ) after being pre-heated by heat exchange with process gas of the combustion process. The combustion gas is pre-heated in a heat exchanger ( 21 ) and/or by means of a surface of the burner lances ( 9, 25 ) which extend through the pre-heating zone ( 8 ), said surface being enlarged to enhance heat exchange.

The invention relates to a method of burning lumps of material, inparticular limestone, in a regenerative shaft kiln with at least twokiln shafts interconnected by an overflow line, in which shafts thecombustion material passes through an upper pre-heating zone, a centralburning zone and a lower cooling zone, wherein combustion air issupplied to the pre-heating zone, cooling air to the cooling zone andcombustion gas, having a low calorific value of less than 7.5 Megajoules (MJ) per m³ _(n) periodically and alternately to each of theshafts through burner lances.

Regenerative shaft kilns of this type, known as MAERZ kilns, distinguishthemselves from shaft kilns of other types particularly being highlycost-effective. It has been established that the increased use of leangas, i.e., a combustion gas with relatively low calorific value, suchas, for example, the by-product in metallurgical works or biogas with alow calorific value of less than 7.5 Mega joules (MJ) per m³, has anadverse effect on the process temperature of the regenerative shaftkiln, and reduces its degree of effective—ness due to the essentiallyhigher flue gas volumes, and its lower temperature. Besides, higher fluegas temperatures require additional measures to prevent overheating ofpost-combustion filtering installations, through flue gas coolers or bymixing fresh air. On the other hand, the use of lean gas leads to betterproduct quality by virtue of the decreased feed of sulphur.

The object of the invention is to find a method of the mentioned typethat avoids the pointed out disadvantages and therefore, enables betterproduct quality and good thermal efficiency T by using a combustion gaswith a relatively low calorific value of less than 7.5 MJ/m³ _(n).

In this invention, this objective is achieved by a method wherein thecombustion gas is pre-heated in heat exchange with the process gas ofthe burning process. Advantageous embodiments of the method of inventionare object of dependent claims, and can be taken from the followingdescription and the drawing. It illustrates a schematic sectionalrepresentation of a regenerative kiln with two parallel shafts.

As already known, lumps of material with a particle size range of e.g.40 to 80 mm are supplied to shafts 1 and 2 of kiln 3, as indicated byarrows 4 and 5. The supply of combustion air, also from the top asindicated by arrow 6 and of combustion gas as per arrow 7 with the helpof the burner lances 9 arranged vertically in the pre-heating zone 8,takes place in intervals of e.g. 15 minutes to shaft 1 and shaft 2alternately. Besides, cooling air is supplied continuously correspondingto arrows 12, 13 from the lower discharge area 10, 11 of the two shafts1, 2 so that burned product after passing the cooling zone 14 can bewithdrawn as cooled in the direction of arrows 15, 16. The flamescreated at the outlets 17 of the burner lances 9 receive pre-heatedcombustion air, because this air flows through lumps of materialpre-heated during the previous operation period. The pre-heating oflumps of material in the pre-heating zone 8 takes place in theneighbouring shaft (2), because the burning gases flow into theneighbouring shaft 2 after passing through combustion zone 18 through anoverflow line 19 and release it as flue gas in the upward direction ofarrow 20.

According to the illustrated embodiment of the invention, thepre-heating of the burning gas takes place in a heat exchanger 21 with ahot gas flow branched off in relatively low volumes from the overflowline 19 through a pipe 22, which is afterwards directed to the flue gasflow indicated by arrow 20. When changing the process mode from shaft 1to neighbouring shaft 2, pipe 23 guided through heat exchanger 21 can beswitched by means of a valve 24 to burner lances 25 of shaft 2.Accordingly it is possible to operate a kiln 3 more economically withlean gas of low calorific value of less than 7.5 MJ/m³ and e.g., of 4MJ/m³.

According to another embodiment of the invention, pre-heating of theburning gas can take place, alternately or additionally to pre-heatingin the heat exchanger 21, in the preheated combustion air of thepre-heating zone, in that at least the outer surface of the burnerlances 9, 25 is designed larger, e.g. by means of ribs.

1. Method of burning lumps of material, in particular limestone, in aregenerative shaft kiln (3) with at least two kiln shafts (1,2)interconnected by an overflow line (19), in which shafts the material tobe burned passes through an upper pre-heating zone (8), a centralburning zone (18) and a lower cooling zone (14), whereby combustion airis supplied in the area of the pre-heating zone (14), cooling air issupplied to the cooling zone (14) and the supply of combustion gashaving a low calorific value of less than 7.5 Mega joules (MJ) per m³_(n) alternates periodically to each of the shafts (1,2) through burnerlances (9.25), characterized in that that the combustion gas ispre-heated in heat exchange with process gas of the burning process. 2.Method according to claim 1, characterized in that the combustion gas inthe preheating zone (8) is heated by the pre-heated combustion airtherein.
 3. Method according to claim 2, characterized in that thecombustion gas is pre-heated while flowing through burner lances (9, 25)extending into the pre-heating zone (8), the surface of the lances beingdesigned enlarged, at least on the outer side.
 4. Method according toclaim 1, characterized in that the combustion gas is pre-heated in heatexchange with a branched off part of the process gas branched off fromthe kiln (3), in that a heat exchanger (21) provided outside the kiln isconnected on one side to a process gas pipe (22) coming from a branchingposition on the kiln and on the other side to a combustion gas pipe (23)leading to burner lances (9, 25).
 5. Method according to claim 4,characterized in that a part of the process gas for pre-heating thecombustion gas is branched off from the overflow line (19) of the kiln.6. Method according to claim 1, characterized in that it is operatedwith combustion gas the lower calorific value of which is less than 5MJ/m³.
 7. Method according to claim 2, characterized in that thecombustion gas is pre-heated in heat exchange with a branched off partof the process gas branched off from the kiln (3), in that a heatexchanger (21) provided outside the kiln is connected on one side to aprocess gas pipe (22) coming from a branching position on the kiln andon the other side to a combustion gas pipe (23) leading to burner lances(9, 25).
 8. Method according to claim 3, characterized in that thecombustion gas is pre-heated in heat exchange with a branched off partof the process gas branched off from the kiln (3), in that a heatexchanger (21) provided outside the kiln is connected on one side to aprocess gas pipe (22) coming from a branching position on the kiln andon the other side to a combustion gas pipe (23) leading to burner lances(9, 25).
 9. Method according to claim 2, characterized in that it isoperated with combustion gas the lower calorific value of which is lessthan 5 MJ/m³.
 10. Method according to claim 3, characterized in that itis operated with combustion gas the lower calorific value of which isless than 5 MJ/m³.
 11. Method according to claim 4, characterized inthat it is operated with combustion gas the lower calorific value ofwhich is less than 5 MJ/m³.
 12. Method according to claim 7,characterized in that it is operated with combustion gas the lowercalorific value of which is less than 5 MJ/m³.
 13. Method according toclaim 8, characterized in that it is operated with combustion gas thelower calorific value of which is less than 5 MJ/m³.
 14. Methodaccording to claim 53, characterized in that it is operated withcombustion gas the lower calorific value of which is less than 5 MJ/m³.